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Bifunctional nanocrystals with integrated plasmonic and catalytic activities hold great promise for analyzing chemical reactions by in situ surface‐enhanced Raman spectroscopy. This Minireview gives a brief introduction to the general strategies for designing such nanocrystals, followed by four typical examples, including their fabrication, characterization, and potential limitation. We then use the reduction of 4‐nitrothiophenol and oxidation of 4‐aminothiophenol as two model systems to demonstrate the capabilities of these bifunctional nanocrystals to monitor chemical reactions for the elucidation of reaction mechanisms and measurement of kinetics. We conclude with perspectives on further development of these bifunctional nanocrystals into a viable platform for investigating other types of catalytic reactions.

 

Facet‐selective etching and deposition, as determined by the landscape of surface energy, represent two powerful methods for the transformation of noble‐metal nanocrystals into nanostructures with complex shapes or morphologies. This review highlights the use of these two methods, including integration of them, for the fabrication of novel monometallic and bimetallic nanostructures with enhanced properties. We start with an introduction to the role of surface capping in controlling the facet‐selective etching or deposition on the surface of Ag nanocrystals, followed by a case study of how to maneuver etching and deposition at different facets of Pd nanocrystals for the fabrication of nanoframes. We then introduce the use of galvanic replacement to accomplish selective etching and deposition on two different facets in an orthogonal manner, transforming Pd nanocubes into Pd−Pt octapods. By complementing galvanic replacement with a chemical reduction reaction, it is also feasible to control the rates of these two reactions for the conversion of Ag nanocubes into Ag@Ag−Au concave nanocubes and Ag@Au core‐shell nanocubes. These transformation methods not only greatly increase the shape diversity of metal nanocrystals but also offer nanocrystals with enhanced plasmonic and/or catalytic properties.

 

Silver is an excellent catalyst for oxidation reactions such as ethylene epoxidation, but it shows limited activity toward reduction reactions. Here we report a strategy to revitalize Ag nanocrystals as a redox catalyst for the production of an aromatic azo compound by modifying their surface with an isocyanide-based compound. We also leverage in situ fingerprint spectroscopy to acquire molecular insights into the reaction mechanism by probing the vibrational modes of all chemical species at the catalytic surface with surface-enhanced Raman spectroscopy. We establish that binding of isocyanide to Ag nanocrystals makes it possible for Ag to extract the oxygen atoms from the nitro-groups of nitroaromatics and then use these atoms to oxidize isocyanide to isocyanate. Concurrently, the coupling between two adjacent deoxygenated nitroaromatic molecules leads to the formation of an aromatic azo compound. 

Silver nanocubes have found use in an array of applications but their performance has been plagued by the shape instability arising from the oxidation and dissolution of Ag atoms from the edges and corners. Here we demonstrate that the shape of Ag nanocubes can be well preserved by covering their edges and corners with a corrosion-resistant metal such as Ir. In a typical process, we titrate a Na 3 IrCl 6 solution in ethylene glycol (EG) into a suspension of Ag nanocubes in an EG solution in the presence of poly(vinylpyrrolidone) (PVP) held at 110 °C. The Ir atoms derived from the reduction of Na 3 IrCl 6 by EG and Ag are deposited onto the edges and then corners for the generation of Ag–Ir core-frame nanocubes. Remarkably, our results indicate that a small amount of Ir atoms on the edges and corners is adequate to prevent the Ag nanocubes from transforming into nanospheres when heated in a PVP/EG solution up to 110 °C. We further demonstrate that these Ag–Ir nanocubes embrace plasmonic properties comparable to those of the original Ag nanocubes, making them immediately useful in a variety of applications. This strategy for stabilizing the shape of Ag nanocubes should be extendible to Ag nanocrystals with other shapes or nanocrystals comprised of other metals. 

We report the fabrication of Ag–Au cuboctahedral nanoboxes enclosed by {100} and {111} facets, respectively, through the orthogonal deposition of Au on two different facets of Ag cuboctahedra. Specifically, we titrate aqueous HAuCl 4 into an aqueous mixture containing Ag cuboctahedra, ascorbic acid, and NaOH (under basic conditions), in the presence of poly(vinylpyrrolidone) (PVP) and cetyltrimethylammonium chloride (CTAC), respectively. In the case of PVP, the oxidation of Ag was initiated from the {111} facets of the cuboctahedra through the galvanic replacement reaction between Au( iii ) and Ag, accompanied by the deposition of Au onto the {100} facets. Because the dissolved Ag( i ) ions could react with NaOH to form Ag 2 O on the {111} facets and thus terminate the galvanic reaction, the Au( iii ) ions would be further reduced by the ascorbate monoanion (HAsc − ) to generate Au atoms for their continuing deposition on the {100} facets, converting Ag cuboctahedra to Ag@Au {100} cuboctahedra. Upon the etching of Ag from the core, we obtained Ag–Au cuboctahedral nanoboxes enclosed by {100} facets. In contrast, when CTAC was present, the oxidation of Ag through a galvanic reaction could continuously proceed on {100} facets as the dissolved Ag( i ) ions would react with the excessive amount of Cl − ions derived from CTAC to produce soluble AgCl 2 − ions rather than insoluble Ag 2 O. As a result, the dissolved Ag( i ) and Au( iii ) ions would be co-reduced by HAsc − for the generation of Ag and Au atoms, followed by their co-deposition onto {111} facets for the generation of Ag@Au {111} concave cuboctahedra. After the removal of Ag from the core by etching, we obtained Ag–Au {111} cuboctahedral nanoboxes enclosed by {111} facets. Both samples of cuboctahedral nanoboxes exhibited strong optical absorption in the infrared region. Interestingly, the cuboctahedral nanoboxes enclosed by {111} facets showed significantly enhanced catalytic activity toward the reduction of 4-nitrophenol by NaBH 4 relative to their counterparts encased by {100} facets. 

We report the fabrication of Ag–Pd concave nanocrystals by introducing the Pd( ii ) precursor into an aqueous suspension of Ag nanocubes in the presence of cetyltrimethylammonium chloride (CTAC) under ambient conditions. Different from the previously reported work that involved the oxidation of Ag and deposition of Pd at random sites on the surface for the generation of Ag–Pd hollow nanocrystals, we demonstrate that the Cl − ions from CTAC can confine the oxidation of Ag atoms to the side faces of a nanocube while the resultant Pd atoms are deposited on the edges in an orthogonal manner. By controlling the amount of the Pd( ii ) precursor involved in a synthesis, we can transform Ag nanocubes into Ag–Pd nanocrystals with different degrees of concaveness for the side faces and controllable Pd contents. We characterize the outermost layer of concave surfaces for the as-obtained Ag–Pd nanocrystals by surface-enhanced Raman scattering (SERS) through the use of an isocyanide probe. This facile approach would enable the fabrication of Ag-based concave nanocrystals for applications in plasmonics and catalysis.